In internal combustion engines it is known to provide a plurality of cylinders and pistons for rotating a drive shaft via a crank shaft. In vehicle applications the crank shaft is coupled to the drive train, which may include a clutch, gearbox, prop shaft (for rear-wheel drive), differential, and final drive shafts. The cylinder comprises inlet and outlet apertures; these inlet and outlet apertures comprise valves for controlling ingress of a fuel/air mixture and egress of exhaust gases. The valves are controlled by one or more camshafts which determine the timing of the opening and closing of the valves with respect to the position or stroke of the piston in the respective cylinder.
It is desirable to adjust several parameters relating to the operation of the valves depending upon the load or requirements placed upon the engine. Such requirements may be, by way of non-limiting example: to increase engine power; and to reduce fuel consumption or engine emissions. Examples of the operational parameters that it is desirable to adjust include:                i. the timing of the valve opening and closing with respect to the piston stroke;        ii. the timing of the inlet valve opening/closing with respect to that of the outlet (exhaust) valve;        iii. the duration of time for which the valves are open and/or closed;        iv. the degree or extent to which the valve is opened (lifted).        
The present invention seeks to provide an improvement in the field of variable camshaft drive mechanisms, which has particular application for vehicles. The invention may be utilised in applications other than for vehicles; for example it is foreseen that the invention may have application in other areas where is it desirable to effect an axial shift of a rotary shaft for example in compressors or pumps.